Cyclic process of treating alkaline pulps



May 19, 1936. I G. A. RICHTER 2,041,666

CYCLIC PROCESS OF TREATING ALKALINE PULPS Original Filed Dec. 4, 1925Patented May 19, 1936 UNITED STATES CYCLIC PROCESS OF TREATING. ALKALINEPULPS George A. Richter, Berlin, N. H., assignor to Brown Company,Berlin, N. H., a corporation of Maine Application December 4, 1925,Serial No. 73,193 Renewed September 26, 1930 23 Claims.

This invention has relation to the production of a.high alpha'cellulosefiber, that is, a fiber containing a high percentage of alpha orresistant cellulose and having certain optimum characteristics, from apulp derived from the digestion of wood chips in an alkaline liquor, andmore particularly in a kraft or sulphate liquor. n W

Wood pulp, aspidduced by an alkaline'diges tion, such as kraft, isespecially characterized by its quality of high strength, which renderssuch pulp industrially valuable for use in the manufacture of strongpapers, e. g., wrapping and bag papers. Such pulp, however, is inferiorin respect of tear resistance and folding endurance,

to the high grade manila or rope pulps, so called, which find extensiveuse in cable insulation papers and in the manufacture of bags, andvarious other products, where the characteristics of'high tearresistance and folding endurance, as well as strength, are of primeimportance.

' Moreover, such alkaline pulps are exceedingly difiicult to bleach.While it is possible to bleach kraft pulp with an excessive amount ofbleach, I

say, about 50% of 35% lime bleach (CaOClz) based on the weight of drypulp, the strength of the resultant bleached pulp is materially loweredand its alpha cellulose content undergoes a material decrease byconversion or degradation into oxycelluloses, so that the commercialvalue of v ,such pulp is lowered. This difliculty in bleaching 72,522filed December 1, 1925, by Milton 0. Schur .and myself, as beingsuccessful in connection with the treatment of sulphite pulp.

In working with or producing various pulps, I

have discovered that if an alkaline digested pulp,

such as kraft, is treated with a relatively con-.-

centrated, cold alkaline liquor, e. g., a solution of caustic soda, I amenabled to produce a fiber having a very high tear resistance and. verygood 50 folding endurance, and which may thus be employed as a manilafiber substitute. The change in the characteristics of the pulp, whichhas been efiected by the alkaline treatment, doubtless arises from thefact that the treatment purifies or frees the pulp from the pentosans,less-resistant celluloses, ligneous and other non-alpha celluloseconstituents contained therein, without perceptibly injuringits alphacellulose content, thereby converting it into a pulp which is especiallyhigh in alpha or resistant cellulose 5 content.

Even after such alkalinetreatment, however,

' tlipiilp does not yield to a bleaching treatment very readily, andrequires an excessive amount of bleach to be converted into a whitepulp, The 10 use of such excessive bleach materially aiiects the fiberstrength of the pulp and the alpha cellulose content thereof, aspreviously stated. I have .further discovered, however, that thenecessity for the use of excessive bleach may be avoided 15 by treatinga thoroughly cooked raw pulp prior to the alkaline treatment, with asmall quantity of oxidizing or chlorinating agent, such as a chlorinesolution. This preliminary treatment of the pulp is not materiallychanged. The chlorine, it appears, reacts with and forms solublecompounds with ligneous and other coloring substances contained in thepulp, or conditions the pulp by rendering such substances moresusceptible to the dissolving or removing reaction exercised thereuponby" the alkaline liquor in the subsequent treatment. After the alkalinetreatment, the pulp may be bleached with a moderate amount of bleachinto a white pulp of high alpha 30 cellulose content suitable for use inthe manufacture of high grade writing papers, or forlother purposeswhere a pulp of high alpha cellulose content is required.

The kraft pulp employed in the present invention may be produced, asordinarily, by the digestion of wood chips (spruce for-example) in adigester, under the requisite time, temperature, and pressure conditionsin accordance with modern practice, in the usual kraft or sulphateliquor 40 containing sodium sulphide, sodium hydroxide, and a relativelyslight or adventitious amount of sodium sulphate. Spruce, hemlock, pine,or any other raw cellulosic material suitable for the production ofkraft pulp may be utilized. After the necessary period of digestion, thedigester contents are blown and the kraft pulp is washed and separatedfrom its accompanying black spent digesting liquor.

The washed pulp is then preferably screened to remove shi'ves, specks,and other contaminating matter, and, if desired, is partially bleachedas hereinbefore described. It is then thickened to the consistencydesired for admixture with a liquor containing the alkaline compoundsem-- ployed for purification. The thickened pulp is then intimatelymixed with a solution containing caustic soda or other equivalentalkaline compounds and is treated therewith, say at room temperature (30C.), for a sufiicient period of time to permit the alkaline solution toreact with and remove the non-alpha cellulose constituents of the pulp.In carrying out the treatment or purification only with caustic soda,too high a concentration of caustic soda must be avoided,

since otherwise the fiber of the pulp will become mercerized (suchmercerization tending to lower the strength of the pulp), and subsequentbeating merely macerates or shreds the pulp, without effecting itshydration, so that paper made therefrom is extremely weak. 0n the otherhand, if too low a concentration of caustic soda is employed, asufiicient purification or reaction upon the impurities contained in thepulp, by the caustic Soda solution, will nottake place and the resultingpulp will not be sufiicientlyhigh in alpha cellulose. In other. words,the concentration of caustic soda employed in the alkaline treatment ismaintained between two limits, namely, be-

tween a maximum concentration at which merjeri aation takes place and aminimum concentra-. tion at which a sufi'minfpurification of the pulp isnot effected. These limits will vary somewhat, depending upon thetemperature or treatment employed and the differences in character ofthe initial raw stock. After the purification, the pulp containing ahigh percentage of alpha cellulose is washed free from its treatingliquor and may be employed as a manila fiber substitute, as previouslyindicated. If desired, the treated or puri- ,40 (CaOC1z),based on thedry "weight of manila fiber.

fied fiber which still possesses the tan or brown.

ish color of the raw kraft pulp may be bleached at a stock consistencyof about 10% to 16% with about 5%. to 19% of 35% lime bleach outmaterially afiecting its strength, tear resist ance, folding endurance,and alpha cellulose content, to a color "which closely approximates orsimulates the light yellow tinge of high-grade In certain cases,however, it is desirable to produce from raw alkaline digested pulps apure white product which is characterized by its high -trati on ofalkali as to cause mercerization.

may be obtained by treating a raw alkaline digested pulp with a solutionof sodium sulphide of somewhat higher concentration than that of asolution of caustic soda. When the caustic soda is replaced by asubstantial proportion of 5 sodium sulphide or in entirety thereby,mercerization of the fiber of the pulp is generally precluded, sincesodium sulphide does not readily cause a mercerization of cellulosefiber.

While I have indicated that the alkaline purii fying or refiningtreatment is ordinarily carried out at room temperature, it may bestated that lower than room temperature (say, 0C. to 25 C.) may beemployed in such treatment. Likewise,'higher temperatures (for example,25 C. 1

to 75 C.) and a correspondingly suitable concentration of alkalinecompound may be employed in such treatment. Thus, in effecting thepurification of kraft pulp with a white liquor utilized for digestingwood chips in a kraft mill, 29

a very good product was obtainedat a treating temperature of 75 C. Inthis connection, I have observed that a more highly concentratedalkaline solution should be associated with a higher treatingtemperature in order to avoid causing 25 a mercerization of the fiberand injury" to the papermaking--characteristics of the pulp Forinstance, in order to treat raw pulp successfully at 30 C. with theusual white liquor of a kraft mill without mercerizing the fiber, suchliquor 30 must be diluted with about water, whereas the treatment, asindicated; may be carried out without the dilution of the liquor at 75C. This observation is in line with the generally known principle thatmercerization may be more readily 35 effected at low-temperatures. Inactual practice, the lower the temperature employed in the purificationtreatment, the greater the care which must be exercised to avoid such ahigh concen- The presenthin vention may be practised advantageously inconnection with a plant wherein raw alkaline digested pulp, such aseither a kraft or soda pulp, is produced. The soda pulp hereinreferredhto is preferably a long-fibered pulp ap- 45 proximatingthe-stmn gth of kraft, i. e., a pulp produced from the digestionoLrawoellulosic material, such as spruce or pine. SuclTkraitor soda pulp maybe employed as the raw matefiahfor-- l the production of a high alphacellulose fiber 5 possessing the properties of very high tearresistance, and folding endurance, and stability.

invention may be obtained from the following description, whenconsidered in conjunction with 5:,

A more complete understanding of the present I dizing agent, such as achlorine solution or bleach liquor, for a relatively sl i ort period oftime prior to treatment with an alkalineilution, and is then treatedwith a solution of a suitablealkaline 0 compound, the pulp issufficiently freed or purialpha cellulose content. If a pulp ofexceptionfied from its non-alpha cellulose constituents to permit thepurified pulp to be bleached with a moderate amount of bleach withoutperceptible injury to its strength or decrease in its ally highwhiteness is desired, such pulp may, if desired, be superbleached in achlorine or other suitable solution.

In certain cases, it may be of advantage to replace a portion or all ofthe caustic soda used in the alkaline treatment by a suitable amount ofother alkaline compound, such as sodium sulphide, which exerts a similarpurifying or refining action on the raw pulp. For instance,substantially equivalent results to those outlined the accompanyingdiagrammatic and conventional drawing, which represents what may betermed a'flow sheet showing the course or fiow of the various materialsemployed inthe process.

The invention will now be described as applied in on connection with thekraft process of fiber liberation, bu t it will be obvious that it maybe, em-

ployed in connection with the soda process.

Referring to the draWing at l, I have indicated a digester in which rawcellulosic material in the 65 form of wood chips may be placed anddigested in an alkaline liquor for the production of an alkaline pulpsuitable for use as a raw material in the preparation of the finishedhigh alpha cellulose fiber, herein desired. After the chips 70 have beencooked in an alkaline cooking liquor comprising a solution of causticsoda and sodium sulphide, under the necessary time, temperature, andpressure conditions in accordance with modern practice, to eifect aliberation of fiber, the

digester contents are blown into a suitable blow pit 2, wherein the pulpis separated from its spent black digesting liquor, which is deliveredfrom the blow pit into a black liquor storage tank 3.

The alkaline raw pulp, together with its assoelated spent black liquor,is withdrawn from the blow pit by a pump 4 and delivered therefrom intoa counter-current washer, consisting of a series of pulp washersindicated conventionally at 5. The washer is preferably of thecounter-current type illustrated and described in U. S. Letters PatentNo. 1,421,664, to Brown Martinson et al., although, if desired, theordinary rotary vacuum type of washer may be employed. In the washer ofthe patent referred to, the pulp is carried continuously through tanksin one direction and the washing liquor is carried successively throughsuch tanks in the opposite direction. The clean wash water is passedfrom a pipe I to the pulp delivered at the last washer of the seriesfrom which the washed pulp is delivered. The washed pulp may bedelivered from the washer by a conduit 9 through the usual screensindicated conventionally at Ill, from which it is delivered to a mixingtank I I, wherein it is intimately mixed with a solution of alkalinecompounds necessary for the purifying treatment. The spent black liquorwhich is associated with the pulp is passed from the first pulp washerof the series by a conduit I3 into the black liquor storage tank 3.

A part of the black liquor from the tank 3 may be withdrawn by a pumpthrough a pipe I4 into the digester I, wherein it is reemployed,together with a freshly prepared cooking liquor, for the digestion ofanother batch of wood chips. The remaining portion is delivered througha pipe I5 into an evaporator I6 wherein a part of the aqueous content ofthe liquor is removed prior to the incineration of its combustiblecontent and the recovery of its inorganic mineral content.

The concentrated liquor from the evaporator I6 is delivered through apipe II into a recovery retort 20 and smelted under reducing conditions,thereby producing a smelt consisting essentially of sodium sulphide andsodium carbonate. For making up losses in the digesting cycle, sodiumand sulphur in the form of salt cake may be introduced into theconcentrated spent liquor from a storage tank I8 through a pipe I9 priordelivered into the water contained in the dissolving tank 2| from whichthe resultant heated smelt liquormay then be passed through the pipe 22into a causticizing tank 23, wherein it may be treated, while beingagitated, with sufi'icient lime to causticize or convert its carbonatecontent to caustic soda. The heated liquor is then filtered as usual, asby passage through a rotary filter 24,

tank 21 may, however, be used for admixture with I30 and by-passed theraw stock in the mixing tank to effect the purification of such stock,as will presently be described. I

Reverting now to the alkaline stock which has been delivered into thetank II, as shown, provision is made for uniformly or intimately mixingsuch stock with asolution containing alkaline compounds, e. g., causticsoda, for its purification. To this end, the stock in the tank istreated,

while being agitated, with a solution containing alkaline purifyingcompounds fed thereinto in controlled and regulated amount by way of afeeding device 30 from a pipe 36. The stock, together with the treatingsolution uniformly admixed therewith, is withdrawn from the mixing tankby a pump 3| and passed into a thickener 32, wherein it is concentratedor thickened to the consistency desired in the purifying treatment, sayabout 6% to 16%. The white water extracted from the stock in thethickening operation is passed from the thickener 32 through a pipe 33into a storage tank 3i from which it is withdrawn by a pump 35 andpassed through the pipe 63 into the mixing tank II, wherein it is reusedfor admixture with other raw alkaline stock to be purified. Thus thewhite water which is removed from the thickening operation andrecirculated to the mixing tank merely serves as a vehicle or carrier.for effecting a thorough mixture and distribution of caustic sodathroughout the stock. Such thorough mixing can only be obtained with adilute stock containing excess carrier which is reemployed in the cycle.The stock is allowed to undergo the purification treatment at relativelylow temperatures, which may range from lower than room temperatures upto C., as previously indicated. When a solution of only caustic soda isemployed, the maximum concentration thereof is below that at which themercerization of the alpha cellulose content occurs, and the minimumabove that at which a suificient purification is not effected. Foroptimum results in respect to alpha cellulose content in the finishedproduct, at a temperature of about 30 C. the minimum and maximumconcentrations in a stock suspension of about 5% or 6% are,respectively, about and caustic soda based on the weight of dry fiber.The alkalinity of the caustic soda solution is thus about 5% to lcaustic soda strength.

At this temperature the stock usually requires about one hour to undergopurification, .or, in other words to complete its passage through bothtreating tanks. Of course; the time of treatment or temperature may bevaried, as desired, de-

pending upon the characteristics of the raw stock. The purified stock,together with the tremendous excess of unused caustic soda contained inthe liquor, is passed through a counter-current washer 38 of a typesimilar to that previously described, wherein it is washed free from itsalkaline constitutents. The washed and purified stock is withdrawn atone end of the washer, through the conduit 39.

The spent liquor which is withdrawn at the other end of the washercontains a large amount of free caustic. Thus, if a caustic solutioncontains 100%caustic soda based on the dry weight of stock at the timewhen it enters the reaction tank 34, approximately 90% of this causticsoda remains unconsumed or unconverted after the purifying treatment hasbeen effected, so that, obviously, the larger portion of the liquor fromthe washer may be 'reutilized for the purification of other raw alkalinestock. As shown, such spent- '4 liquor is passed from the washer throughthe con duit 40 into an intermediate storage tank 4| from which-it iswithdrawn by a pump 42 and deliveredwthrbugh a conduit 36 into thefeeding device 307which serves to regulate the supply of purifyingsolution going to;the mixing tank I I. In order to avoid an accumulationof organic matter which has been removed from the raw alkaline stock,

however, provision is made to divert a certain small portion of spentliquor obtained from the counter-current washer 38, either to theevaporated liquor prior to its incineration, of for use together withalkaline liquor for the liberation passed into the digester I, ifdesired. This volume diverted for evaporation or use in the digester mayapproximate 10% of the total spent liquor.

For the replenishment of losses occasioned in the supply of alkalinecompounds used in the purifying treatment, provision is made forcontinuously removing a certain small portion of strong, hot whiteliquor from the storage tank 21 to make up such losses. To this end, Ihave shown a pump 50 which withdraws a portion of such hot white liquorfrom the tank 21 and forces it through a pipe and a cooler 52, thencethrough a pipe 53 into the intermediate storage tank M, from which it issupplied to the feeding device 30 for admixture with raw alkaline stock.Such an arrangement -,allows the process to be carried out in averyeconomical manner. Thus, when the purifying treatment is carried outin connection with a raw alkaline pulp derived from the digestion ofwood chips by the kraft process, as outlined, the treating or purifyingliquor for such alkaline pulp will contain a certain portion of sodiumsulphide as well as sodium hydroxide. Sucha liquor has been found toserve the purpose of. converting kraft stock to a high alpha" cellulosecontaining fiber possessing the propertie spf very'high tear resistanceand folding en- However, when the process is carried outpin aplantwherein chips are digestedin a solution of sodium hydroxide,obviously, then, the

"purifying treatmentis carried out with caustic soda alone as thepurifying reagent.

However, in a kraft pulp plant favored by a cheap electrical supply sothat sodium hydroxide maybe manufactured economically, that is, wherelocal and economic conditions permit, a modification of the hereinbeforedescribed procedure may be advantageous. The raw materials for carry ingout the process insuch case may consist of scdiumhydroxide and sulphur,rather than salt gether with the concentrated spent liquor. Of

' course, the amount of sodium constituent diverted to .the kraft millfrom the purifying treatment will be-sufficient to make up sodium lossesoccasioned therein. With such an arrangement,

' even. though the purification treatment of the alkaline pulp ispractised'in connection with the kraft process, no sodium sulphide willbe present in the purifying liquor. 'Under conditions where thepurifying treatment is carried out with sodium hydroxide alone, theresulting purified fiber is more easily bleachable, especially when ithas been pretreated with an oxidizing'agent.

In certain other cases it may be. of advantage to change the describedprocedure to one wherein all the spent liquor obtained from-the washer38 is employed, instead of the filtered hot white liquor, for thecooking of wood chips in the digester I. In other words, the proceduremay be one wherein all of the liquor from the white liquor storage tank21 is employed as a purifying liquor, and all of the liquor from thepurifying treatment is utilized inthe digester I for the liberation ofraw fiber from chips. Inasmuch as only a small proportion of the totalalkaline content in the white liquor item the counter-current washer 38has actually been converted to inert or inactive constituent in thepurifying treatment, the strong liquor from the washer 38 hassubstantially the same concentration of sodium sulphide and canstic sodaunder certain conditions hereinbefore stated, as the white liquor in thestorage tank 21. This being the case, it may be used in the digester Ifor cooking fresh chips, rather than recirculated to the mixing tank II,as shown in the drawing. With such an arrangement, the hot white liquorfrom the storage tank 21 may be passed through a cooler prior to itspassage through the control feeding device 30 and into the mixing tank II, if low temperature is employed in the purification treatment. On theother hand, if a full strength white liquor, with which a treatingtemperature of 75 C, should be associated, is used in the purificationtreatment, it is, of course, employed without cooling.

Among the advantagesflowing from the practice of the hereinbeforedescribed procedure, is that I am able to produce, with a high yield,from raw alkaline digested pulp, a fiber having a high alpha cellulosecontent and possessing a very high tear resistance and foldingendurance. The

Kraft pulp Purified pulp Mullen strength (beaten) 155-160 120-142. '1"220-245 350-400. Alpha cellulose, 7 88-89 93-96. Pentosans, 7-10 3-5.Folding endurance 1800-2400 3000-4000 double fold Yield of process.92-95.

From the foregoing table, it will be seen that ment prior to thealkaline treatment or purification, as hereinbefore indicated. Suchpretreatment conditions the pulp for the alkaline treatment and allowsthe pulp to be bleached after the alkaline treatment with a moderateamount of bleach and without perceptible injury to its strengthordecrease in its alpha cellulose content. The pretreatment is, however,apt to lower the yield of final product somewhat. In'such case, the pulpis pretreated approximately at a.

stock density of about preferably with a solution containing 1% to 2%chlorine or 2% to 6% of 35% lime bleach (CaOCl-z) based on the dryweight of fiber. The oxidizing agent reacts with the difilcultlyremovable non-alpha cellulose constituents contained in the pulp,forming soluble reaction products, or products more easily removable bythe subsequent alkaline treatment. The pretreated stock isthcn purifiedin .a solution of alkaline compounds, as hereinbefore described.

Since the pretreatment and subsequent alkaline purifying treatment haveremoved from the raw alkaline digested pulp a substantial amount ofnon-alpha cellulose constituents, the purified pulp can be bleached withmaterially less hypochlorite or bleach than is required without suchtreatments, to produce a pure white product; and this bleachingoperation may be effected without material injury to the strength of thepurified pulp, or reduction of its alpha cellulose content. Ordinarily,a bleaching treatment at a stock consistency of about to 16% with about5% to 10% of 35% lime bleach (CaOClz) based on the dry weight of pulp,is sufficient to bring the pulp up to a whiteness equivalent to orapproximating that of ordinary bleached sulphite pulp.

The stock thus bleached may be improved in color by a superbleachingoperation, which may be carried out at a stock consistency 'of from 4%to 6%, with about 2% to .5% chlorine, based on the weight of dry fiber.

Owing to its properties resulting from the purifying treatment which ithas received, the white high alpha cellulose fiber may be employed notonly as a substitute for new rag fiber in the manufacture of high gradebond and ledger papers, but also in the manufacture of the celluloseesters and the cellulose ethcrs, and for other purposes where asubstantially pure cellulose fiber is required.

Thus, by the practice of this invention, I am enabled to increase thetearresistance and folding endurance of the raw alkaline digested pulpat the expense of its Mullen strength, and also to purify it into aproduct which is especially high in alpha cellulose. This is highlydesirable, since the tear resistance and its accompanying quality offolding endurance are of extreme importance in high grade papers,whereas the Mullen or bursting strength is usually ofsecondaryimportance. This may be better understood from the fact that it isrelatively easy to increase the Mullen or bursting strength of a papersheet by suitable beating and sizing. The heating and sizing operations,however, do not affect the tear test so appreciably, so that it is adistinct ad-- vantage to increase the tear test even at the expaper maybe employed for the production 01' artificial reed fiber, such as servesin the manufacture of baby carriages, furniture, etc. Such a papershould have a high tear resistance, should be soft and yet firm, andshould have sufficient porosity to permit its ready impregnation withglue, varnish, or other waterproofingreagents. Such paper can beproduced from the product resulting from the herein described process.

Before concluding, it may be stated that, while I have specified certainconcentrations of stock and treating compounds in order to more fullyand adequately describe my invention, these concentrations may vary,depending upon plant equipment and other conditions. That is to say, theplant equipment with which the present invention was practiced permittedthe treatment of an alkaline digested stock at a concentration of about5% to 6%, most efliciently and successfully. On the other hand, a stockconcentration of 10% or greater may be treated under other plantconditions. In such cases, the percentage of caustic soda or otheralkaline compounds utilized in the purification may be correspondinglyadjusted. The determining factors in carrying out the purifyingtreatment or reaction are the concentration of the alkaline compounds inthe liquor and the temperature at which the liquor is used. Of course,if sodium sulphide or some other reagent, which is less drastic oreffective in its reaction than caustic soda, is used, the percentage ofsuch reagent may be increased in order to produce a productsubstantially equivalent to that obtained by the use of caustic soda.

The change in the characteristics of the raw alkaline digested pulp iseffected with a relatively high yield of finished high alpha celluloseproduct, the yield ranging from 93% to 95%, based on the weight of dry,raw pulp.

At the risk of repetition, it may be stated that the drawing is whollydiagrammatic and conventional, no attempt having been made to show thetrue size relationship between the various instrumentalities, or theirproper location. For the sake of clarity in the drawing, certain detailsof construction have been omitted, but it will of course be understoodthat fittings, pipes, pumps, devices for controlling temperatureconditions; or any other apparatus necessary for maintaining the flow ofthe various materials or for maintaining certain conditions, will besupplied wherever necessary.

I do not herein claim the subject matter of my Having thus describedthis invention and a method of practicing the same, it should be obviousto those skilled in the art that various changes and modifications inprocedure might be resorted to without departing from its spirit orscope as defined by the appended claims.

What I claim is:

1. A process which comprises treating with a liquor containing freshalkaline sodium compounds, pulp derived from the digestion of wood chipsin a digesting liquor containing sodium compounds, reutilizing oneportion of alkaline liquor resulting from such treatment in the alkalinetreatment of other pulp,- and smelting and recovering the sodiumcompounds from the other portion of muchliquor, together with the sodiumcompounds of said spent digesting liquor, in the form of alkaline sodiumcompounds.

' 2. A process which comprises treating with a liquor containing freshalkaline sodium compounds, pulp derived from the digestion of wood chipsin a digesting liquor containing sodium compounds, reutilizing oneportion of alkaline liquor resulting from such treatment in the alkalinetreatment of other pulp, smelting and recovering the sodium compoundsfrom the other poition of such liquor, together with the sodiumcompounds of said spent digesting liquor, and'adding suflicient alkalinesodium compounds to said first mentioned portion to carry out thealkaline treatment of other pulp.

3. A process which comprises treating with a liquor containing freshalkaline sodium compounds, pulp derived from the digestion of wood chipsin a digesting liquor containing sodium compounds, reutilizing oneportion of alkaline liquor resulting from such treatment in the alkalinetreatment of other pulp, smelting and recovering the sodium compoundsfrom the other portion of such liquor, together with the sodiumcompounds of said spent digesting liquor, in the form of alkaline sodiumcompounds, and adding suflicient alkaline sodium compounds from therecovered alkaline sodium compounds to said first mentioned portion tocarry outithe alkaline treatment of other pulp.

4. A-process which comprises treating with a liquor containing freshalkaline sodium compounds, raw pulp derived from the digestion of woodchips in a digesting liquor containing alkaline sodium compounds,utilizing alkaline liquor resulting from such treatment as digestingliquor for the liberation of other raw pulp from wood chips, smeltingand recovering the sodium compounds from said digesting liquor in theform of alkaline sodium compounds, and utilizing the recoveredcompoundsin an alkaline liquor for. the treatment of other raw pulp.

5. In purifying unbleached pulp preparedfrom wood by an alkalineprocess, the sequence of operations which comprises partially bleachingthe pulp, treating the-partially bleached pulp with a solution ofcaustic alkali, the. pulp and solution being brought together-in suchamounts and concentrations as to give a resulting suspension hav-.

ingan alkalinity equivalent to that of a caustic soda solution of atleast about 5% strength but below mercerizing strength, continuing thetreatment for the desiredperiod of time at a temperature less than theboiling point of water, separating caustic "alkali solution from, thesotreated pulp and washing the latter, subjectingthe treated and washedpulp in aqueous 'suspension to a second stage of bleaching treatment,and washing the resulting finally bleached pulp.

6. A process which comprises subjecting unbleached alkaline wood pulp toa preliminary bleaching treatment with a fraction of the total bleachingagent required, admixing the partially bleached pulp with a solution ofcaustic alkali of such concentration and in such' proportion that theresulting mixture has an alkalinity equivalent to a solution of causticsoda of at least about 5% strength but below mercerizing strength,

maintaining the mixture at a temperature below- 212 F. for the desiredperiod of time, separating caustic alkali solution from said mixture andwashing the pulp, subjecting the washed pulp to a secondary bleachingtreatment with the restof the bleaching agent required, and washing theresulting pulp.

'7. A process which comprises subjectingun-j" bleached sulphate pulpobtained from coniferous.

wood to a preliminary bleaching treatment with a fraction of the totalbleaching agent required,

period of time, separating caustic alkali solution from the mixture and.washing the pulp, subjecting the washed pulp to a secondary bleachingtreatment with the rest of the bleaching agent required, and washing theresulting pulp.

8. A process which comprises subjecting unbleached sulphate pulpobtained from coniferous wood to a preliminary bleaching treatment witha fraction of the total bleaching agent required, thickening thepartially bleached pulp, admixing therewith caustic soda solution insuch amount and concentration as to bring the re- 'sulting mixture to aconsistency of about 6% of' dry pulp substance with an alkalinity of theresulting solution equivalent to about 6.5% of caus-, tic soda,maintaining the mixtureat lessthan the boiling point of water for aboutone hour with agitation, separating caustic soda solution .from

the mixture and washing the pulp, subjecting the washed pulp to asecondary bleaching treatment with the rest of the bleaching agentrequired, and washing the resultingbleached pulp. I

9. A method for the treatment of. cellulosic materials which comprisesreducing the same to the condition of pulp by cooking with an -aqueoussolution containing an, alkaline sulphide, removing resulting solutionfrom the remaining pulp,

and treating thelatter with an aqueous alkaline solution of a reactivecompound of sulphur.

.10. A method for the treatment of cellulosic' materials which comprisesreducing the same to the condition of pulp by efiecting incomplete]cooking with anaqueous solution containing an alkaline sulphide,removing the resulting solution from the pulp and treating the latterwith, 'a'

reactive compound of sulphur in alakaline s'olution.

11. In purifying unbleached chemical wood pulp, the sequence ofoperations which comprises partially bleaching-the pulp, washing thepartially bleached pulp, treating the partially bleached and washed pulpwith a solution of caustic alkali, the pulp and solution being broughttogether in such amounts and concentrations as to give a resultingsuspension having an alkalinity equivalent to that of a caustic sodasolution of from about 5% to about 12% strength and a pulp concentrationof from, about 5% to about 10%, continuing the treatment'for from about10 minutes to about 6- hours, separating caustic alkali solution, ,fromthe so-treated pulpv and washing the latter, subjecting the treated andwashed pulp to a second stageof bleaching treatment, and washing theresulting finally bleached pulp.

12. A method for the treatment of cellulosic materials which comprisesreducing the same to the condition of pulp by cooking with an aqueoussolution containing an alkaline sulphide, removing resulting solutionfrom the remaining pulp, and treating the latter with an aqueousalkaline solution of sodium sulphide.

alkaline sulphide, removing the resulting solution from the pulp, andtreating the latter with sodium sulphide in alkaline solution.

14. A method for the treatment of cellulosic materials which comprisesreducing the same to the condition of pulp by cooking with an aqueoussolution containing an alkaline sulphide, remov ing resulting solutionfrom the remaining pulp, treating the pulp with a bleaching agent, andthen treating the pulp with an aqueous alkaline solution of a reactivecompound of sulphur.

15. A method for-the treatment of cellulosic materials which comprisesreducing the same to the condition of pulp by cooking with an aqueous'solution containing an alkaline sulphide, removing resulting solutionfrom the remaining pulp, treating the pulp with a bleaching agent inamount insuflicient to bring the pulp to substantial whiteness, thentreating the pulp with an aqueous alkaline solution of a reactivecompound of sulphur, and again treating the pulp with suflicientbleaching agent to bring it to substantial whiteness.

16. A method for the treatment of cellulosic materials which comprisesreducing the same to the condition of pulp by cooking with an aqueoussolution containing an alkaline sulphide, removing the resultingsolution from the remaining pulp, treating the pulp with a bleachingagent, and then treating the pulp with an aqueous alkaline solution 01'sodium sulphide.

17. A method for the treatment of cellulosic materials which comprisesreducing the same to the condition of pulp by cooking with an aqueoussolution containing an alkaline sulphide, removing resulting solutionfrom the remaining pulp, treating the pulp with a. bleaching agent inamount insufilcient to bring the pulp to substantial whiteness, thentreating the pulp with an aqueous alkaline solutionoi sodium sulphide,and again treating the pulp with suflicient bleaching agent to bring itto substantial whiteness.

18. A method-for the treatment of cellulosic materials which comprisesreducing the same to the condition otpulp by cooking with an alkalineliquor containing caustic soda, removing resulting liquor from theremaining pulp, and treating the latter with an aqueous alkalinesolution of reactive compound of sulphur.

19. A method for the treatment of cellulosic materials which comprisesreducing the same to the condition of pulp by cooking with an alkalineliquor containing caustic soda, removing resulting liquor from theremaining pulp, and treating thelatter with an aqueous alkaline solutionof sodium sulphide.

20. A method for the treatment of cellulosic materials which comprisesreducing the same to die condition of pulp by cooking with an alkalineliquor containing caustic soda, removing resulting liquor from theremaining pulp, treating the pulp with a bleaching agent, andthen-treating the pulp with an aqueous alkaline solution of a reactivecompound of sulphur.

21. A method for the treatment of cellulosic materials which comprisesreducing the same to the condition of pulp by cooking with an alkalineliquor containing caustic soda, removing resulting liquor from theremaining pulp, treating the pulp with a bleaching agent in amountinsumcient to bleach the pulp to substantial whiteness, then treatingthe pulp with an aqueous alkaline solution of a reactive compound ofsulphur, and again treating the pulp with suflicient bleaching agent tobleach it to substantial whiteness.

22. A method for the treatment of cellulosic materials which comprisesreducing the same to the condition of pulp by cooking with an alkalineliquor containing caustic soda, removing resulting liquor from theremaining pulp, treating the pulp with a bleaching agent, and thentreating the pulp with an aqueous alkaline solution of GEORGE A.RICHTER.

